The molecular basis of terminal nephron differentiation, the process by which immature renal epithelial cells exit the cell cycle and express physiological functions, remains obscure. Failure of terminal nephron differentiation results in renal dysplasia, cystogenesis and cancer. Thus, a better understanding of the transcriptional framework which regulates renal cell differentiation is of great clinical significance. The tumor suppressor protein, p53, is a member of a family of transcription factors that regulate cellular growth, differentiation, senescence and apoptosis, and play a key role in cell fate determination during development and in response to stress. During the previous funding period, we demonstrated that p53 and its homologue p73 act as master transcriptional regulators upstream of genes required for terminal nephron differentiation;this function of p53/p73 is accomplished via cooperation with other developmentally regulated transcription factors and recruitment of co-activators or co-repressors. Consistent with this role, p53 -/- mice exhibit defects in terminal nephron differentiation, which can be reproduced by kidney-specific deletion/inactivation of p53. The overall goal of this competing grant renewal is to elucidate the mechanisms governing p53 activation and commitment to promote the terminal differentiation fate in the developing kidney. We will use a variety of in vivo genetic and biochemical approaches, together with an in vitro organ culture system, to test a model in which Mdm2 and other negative regulators of p53 exert strict spatiotemporal control on p53 during terminal nephron differentiation. We propose that p53 is preferentially required to execute a differentiation program in the ureteric bud cell lineage, and that this function is orchestrated and deciphered by distinct acetylation and phosphorylation cassettes which differentially influence gene expression patterns and thus cell fate. Investigations of how p53 is regulated during development and how p53 executes its differentiation functions are important basic steps toward understanding the genetic control of organogenesis. The proposed studies have important clinical implications as abnormal renal development is the leading cause of chronic renal failure in infants and children. PUBLIC HEALTH RELEVANCE: This proposal addresses a critical step in kidney development related to the cellular and molecular events that control the transition of immature kidney cells to become differentiated and functional. Abnormalities of kidney development (i.e., congenital malformations) account for up to 40% of cases of chronic kidney failure in infants and children requiring dialysis and transplantation. Understanding how kidney epithelial cells transition from an immature to a more differentiated fate should provide insights into novel treatments of important diseases including kidney malformations, cancer, and recovery from acute kidney injury.